(RFR)(S)(10): 8176768: hotspot ignores PTHREAD_STACK_MIN when creating new threads

Thomas Stüfe thomas.stuefe at gmail.com
Tue Mar 21 08:08:38 UTC 2017 

Hi Chris,

looks fine to me.

small nit: os_linux_cpp:

+  // In that cast just subract the page size to get the maximum possible
stack size.

subract->substract

I would have preferred SIZE_MAX instead of UINT_MAX, but both is fine to me.

--

As for the size_overflow problem, I wonder whether we are overthinking
this. Maybe a more pragmatic approach would have been to just define a
reasonable upper maximum for stack sizes which could be well below
UINT_MAX. Somehow I cannot see users wanting to start threads with more
than ~4GB of thread stack size.

Kind Regards, Thomas



On Tue, Mar 21, 2017 at 12:29 AM, Chris Plummer <chris.plummer at oracle.com>
wrote:

> On 3/17/17 11:37 PM, Chris Plummer wrote:
>
>> On 3/17/17 8:17 PM, Chris Plummer wrote:
>>
>>> On 3/17/17 7:01 PM, David Holmes wrote:
>>>
>>>> On 18/03/2017 9:11 AM, Chris Plummer wrote:
>>>>
>>>>> Looks like this will need some more work since the added asserts are
>>>>> triggering on mac os x (which is the only place we'd currently expect
>>>>> them to assert).
>>>>>
>>>>> The problem is that the code I found that rounds up to the page size is
>>>>> only applied to java threads created by the VM for which the java user
>>>>> specified no stack size. The VM and Compiler thread sizes are not
>>>>> rounded. The failure I saw was with
>>>>> runtime/CommandLine/OptionsValidation/TestOptionsWithRanges.java when
>>>>> is
>>>>> specified -XX:CompilerThreadStackSize=9007199254740991. I hit the
>>>>> assert
>>>>> with an EINVAL. The size is not aligned, but it could also be
>>>>> complaining because it is too big. I haven't tried aligning it yet to
>>>>> see.
>>>>>
>>>>> On Linux we do the following:
>>>>>
>>>>>   stack_size = align_size_up(stack_size +
>>>>> os::Linux::default_guard_size(thr_type), vm_page_size());
>>>>>
>>>>> We don't do this on BSD. I think the same on BSD would solve this
>>>>> problem. I'm not sure about adding the guard size. I'll need to see if
>>>>> mac os x has the same pthread bug as linux does.
>>>>>
>>>>
>>>> At this stage I would only deal with alignment issues. IIRC the guard
>>>> issue only affected Linux.
>>>>
>>> Yes, that's what I eventually concluded. I put the fix in
>>> os::Posix::get_initial_stack_size() in os_posix.cpp, and only did the
>>> page aligning, not add the guard page. That way all Posix ports are fixed
>>> in one place. It seems to be working.
>>>
>>>>
>>>> BTW, did you know java users can specify the size of the a new thread's
>>>>> stack:
>>>>>
>>>>
>>>> Yes I mentioned that in another reply - wondering whether we suitably
>>>> check and aligned such requests.
>>>>
>>> No we don't. Below I mentioned I was able to trigger the assert with a
>>> 257k stack size. I guess I wasn't clear that I did that from Java. I have a
>>> new test to add that will be testing this, plus the 9007199254740991 stack
>>> size (which fails to create the thread with an OOME, but that's
>>> acceptable). The fix I mention above in os::Posix::get_initial_stack_size()
>>> takes care of this issue also.
>>>
>> Rounding up triggers a new assert, this time on 32-bit x86 and arm.
>>
>> I should have clarified it's 9007199254740991 "K", which is
>> 9223372036854774784. Unfortunately on 32bit systems that is asserting with
>> EINVAL. I think we need to do a better job of dealing with 32-bit size_t
>> values:
>>
>> jlong java_lang_Thread::stackSize(oop java_thread) {
>>   if (_stackSize_offset > 0) {
>>     return java_thread->long_field(_stackSize_offset);
>>   } else {
>>     return 0;
>>   }
>> }
>>
>>       jlong size =
>> java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));
>>       // Allocate the C++ Thread structure and create the native thread.
>> The
>>       // stack size retrieved from java is signed, but the constructor
>> takes
>>       // size_t (an unsigned type), so avoid passing negative values
>> which would
>>       // result in really large stacks.
>>       size_t sz = size > 0 ? (size_t) size : 0;
>>       native_thread = new JavaThread(&thread_entry, sz);
>>
>> 9223372036854774784 is 0x7ffffffffffffc00 (close to 64 bit MAX_INT),
>> which is 0xfffffc00 when cast to a size_t on a 32-bit system (close to
>> 32-bit MAX_UINT). Round it up to the 4k page size and you get 0, which I
>> guess pthread_attr_setstacksize() doesn't like. So I think more processing
>> of the size is needed here. Maybe in os::create_thread() we should check
>> for 0 after rounding up, and subtract the os page size if it is 0. However,
>> I think we should also avoid truncating on 32-bit to what is basically some
>> random number. Maybe if "size" (a jlong) is greater than UINT_MAX, then
>> just set "sz" (a size_t) it to UINT_MAX.
>>
>> Ok, I think I have this all worked out now. I've added fixes for
> unaligned stack sizes, 32-bit truncating of stack size, and the "aligning
> up to 0" problem. I also added a test. Here's the latest webrev:
>
> http://cr.openjdk.java.net/~cjplummer/8176768/webrev.02/webrev.hotspot
>
> Here's what's changed since webrev.01:
>
> os_posix.cpp: In os::Posix::get_initial_stack_size(), first round up the
> stack size to be paged aligned. This fixes issues on Mac OS X (other
> platforms seem to be immune to this). Then check if the size is zero after
> rounding up to the page size. Subtract the page size in this case to
> produce the maximum stack size allowed. Surprisingly I got no complaint
> from gcc for subtracting from an unsigned value that is known to be 0.
>
> os_linux.cpp: In os::create_thread(), I also check here to make sure the
> size is not 0 after adding the guard page and aligning up, and subtract the
> os page size if it is 0.
>
> jvm.c: In JVM_StartThread(), on 32-bit platforms if the size is greater
> than UINT_MAX, then I set the size to UINT_MAX. Note it will later be
> rounded up to 0 in os::Posix::get_initial_stack_size(), which will result
> in subtracting the os page size to get the actual maximum allowed stack
> size.
>
> TooSmallStackSize.java: added test case for unaligned stack sizes.
>
> TestThreadStackSizes.java: New test. Creates new threads with every size
> up to 320k in 1k increments. Then creates threads with a few other sizes
> that can be problematic.
>
> thanks,
>
> Chris
>
> Chris
>>
>>
>>
>>>>     public Thread(ThreadGroup group, Runnable target, String name,
>>>>>                   long stackSize) {
>>>>>         init(group, target, name, stackSize);
>>>>>     }
>>>>>
>>>>> Fortunately we already force the stackSize to be at least
>>>>> _java_thread_min_stack_allowed. However, we don't do any OS page
>>>>> rounding on Mac OS X as noted above, and I was able to trigger the
>>>>> assert by creating a thread with size 257k.
>>>>>
>>>>
>>>> Note this means that OSX stack logic is broken because it will
>>>> currently be silently failing due to EINVAL!
>>>>
>>> Yes, that is correct.
>>>
>>> Chris
>>>
>>>>
>>>> I'll get another webrev out once I've made the needed fixes. I also have
>>>>> a new test I'd like to add.
>>>>>
>>>>
>>>> Ok.
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>> Chris
>>>>>
>>>>> On 3/16/17 9:27 PM, Chris Plummer wrote:
>>>>>
>>>>>> Ok, time for a new webrev:
>>>>>>
>>>>>> http://cr.openjdk.java.net/~cjplummer/8176768/webrev.01/webr
>>>>>> ev.hotspot
>>>>>>
>>>>>> The only thing that has changed since the first webrev are the asserts
>>>>>> added to os_linux.cpp and os_bsd.cpp. And to summarize what we discuss
>>>>>> already:
>>>>>>
>>>>>>  - The assert should never happen due to the stack size being too
>>>>>> small since it will be at least PTHREAD_STACK_MIN.
>>>>>>  - The assert should never happen due to an unaligned stack size
>>>>>> because we always align it to the page size.
>>>>>>  - Any assert would therefore be a VM bug and not due to user error.
>>>>>>  - No fixing the java launcher. If the user specifies a stack that is
>>>>>> too small, hotspot will already detect this. If the user specifies a
>>>>>> stack size that is large enough but not page aligned, then we just
>>>>>> ignore any error (if the platform doth protest) and the user gets a
>>>>>> main thread with a stack size set to whatever the OS default is.
>>>>>>
>>>>>> I still need to retest (I only ran TooSmallStackSize.java), but figure
>>>>>> getting agreement on the changes first would be best before I bog down
>>>>>> our testing resources.
>>>>>>
>>>>>> thanks,
>>>>>>
>>>>>> Chris
>>>>>>
>>>>>> On 3/15/17 10:03 PM, Chris Plummer wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> Please review the following:
>>>>>>>
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8176768
>>>>>>> http://cr.openjdk.java.net/~cjplummer/8176768/webrev.00/webr
>>>>>>> ev.hotspot
>>>>>>>
>>>>>>> While working on 8175342 I noticed our stack size on xgene was 8mb
>>>>>>> even though I was specifying -Xss72k. It turns out the following code
>>>>>>> was failing:
>>>>>>>
>>>>>>>       pthread_attr_setstacksize(&attr, stack_size);
>>>>>>>
>>>>>>> Although we computed a minimum stack size of 72k, so -Xss72k should
>>>>>>> be fine, pthreads on this platform requires the stack be at least
>>>>>>> 128k, so it failed the pthread_attr_setstacksize() call. The end
>>>>>>> result is pthread_attr_setstacksize() had no impact on the thread's
>>>>>>> stack size, and we ended up with the platform default of 8mb. The fix
>>>>>>> is to round up the following variables to PTHREAD_STACK_MIN after
>>>>>>> computing their new values:
>>>>>>>
>>>>>>>       _java_thread_min_stack_allowed
>>>>>>>       _compiler_thread_min_stack_allowed
>>>>>>>       _vm_internal_thread_min_stack_allowed
>>>>>>>
>>>>>>> For solaris, there was an issue using PTHREAD_STACK_MIN. You need to
>>>>>>> #define _POSIX_C_SOURCE >= 199506L in order to get PTHREAD_STACK_MIN
>>>>>>> #defined, and this needs to be done before including OS header files.
>>>>>>> I noticed that on solaris we were using thr_min_stack() elsewhere
>>>>>>> instead of PTHREAD_STACK_MIN, so I decided to do the same with this
>>>>>>> fix. Either way is ugly (the #define or using thr_min_stack()).
>>>>>>>
>>>>>>> And speaking of the existing use of thr_min_stack(), I deleted it. It
>>>>>>> was being applied before any adjustments to the stack sizes had been
>>>>>>> made (rounding and adding red, yellow, and shadow zones). This mean
>>>>>>> the stack ended up being larger than necessary. With the above fix in
>>>>>>> place, we are now applying thr_min_stack() after recomputing the
>>>>>>> minimum stack sizes. If for any reason one of those stack sizes is
>>>>>>> now too small, the correct fix is to adjust the initial stack sizes,
>>>>>>> not apply thr_min_stack() to the initial stack sizes. However, it
>>>>>>> looks like no adjustment is needed. I did something close to our
>>>>>>> nightly testing on all affect platforms, and no new problems turned
>>>>>>> up.
>>>>>>>
>>>>>>> thanks,
>>>>>>>
>>>>>>> Chris
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>
>>
>

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